Mr Kim told MPs at a parliamentary committee meeting that the missile has "considerable range" but not enough to hit the US mainland.

The range he described could refer to a mobile North Korean missile known as the Musudan, believed to have a range of 3000 kilometres (1800 miles).

That would make Japan and South Korea potential targets - with US bases in both countries - but there are doubts about the missile's accuracy.

The Pentagon announced that it will deploy a missile defence system to the US Pacific territory of Guam to strengthen regional protection against a possible attack.

Experts say North Korea has not demonstrated that it has missiles capable of long range or accuracy. Some suspect that long-range missiles unveiled by Pyongyang at a parade last year were actually mock-ups.

"From what we know of its existing inventory, North Korea has short- and medium-range missiles that could complicate a situation on the Korean Peninsula (and perhaps reach Japan), but we have not seen any evidence that it has long-range missiles that could strike the continental US, Guam or Hawaii," James Hardy, Asia Pacific editor of IHS Jane's Defence Weekly, wrote in a recent analysis.

Kim Kwan-jin said that if North Korea was preparing for a full-scale conflict, there would be signs including the mobilisation of a number of units, including supply and rear troops, but South Korean military officials have found no such preparations.

"(North Korea's recent threats) are rhetorical threats. I believe the odds of a full-scale provocation are small," he said. But he added that North Korea might mount a small-scale provocation such as its 2010 shelling of a South Korean island, an attack that killed four people.

Pyongyang has been railing against joint US and South Korean military exercises taking place in South Korea and has expressed anger over tightened UN sanctions for its February nuclear test. Many of the threats come in the middle of the night in Asia - daytime for the US audience.

Analysts say the threats are probably efforts to provoke softer policies from South Korea, to win diplomatic talks with Washington and to solidify the image of young North Korean leader Kim Jong Un.

At times, Pyongyang has gone beyond rhetoric.

On Tuesday, it announced it would restart a plutonium reactor it had shut down in 2007. A US research institute said yesterday that satellite imagery shows that construction needed for the restart has already begun.

For a second day today, North Korean border authorities denied entry to South Koreans who manage jointly run factories in the North Korean city of Kaesong. South Koreans already at the plant were being allowed to return home.

South Korea has prepared a military contingency plan should North Korea hold South Korean workers hostage in Kaesong, Defence Minister Kim said.

Outraged over comments in the South about possible hostage-taking and a military response from Seoul, a North Korean government-run committee threatened to pull North Korean workers out of Kaesong as well.

North Korea's military statement today, from an unnamed spokesman from the General Bureau of the Korean People's Army, said its troops had been authorised to counter US "aggression" with "powerful practical military counteractions," including nuclear weapons.

It said America's "hostile policy" and "nuclear threat" against North Korea "will be smashed by the strong will of all the united service personnel and people and cutting-edge smaller, lighter and diversified nuclear strike means."

US Defence Secretary Chuck Hagel said Washington is doing all it can to defuse the situation.

South Korea's Defence Ministry said its military is ready to deal with any provocation by North Korea.

The defence minister, however, was criticised by MPs over a North Korean defector who stole a South Korean fishing boat last night and fled back to North Korea across the western sea border.

Kim said South Korean radar had a "blind spot" in the area and South Korean troops were unaware the defector was fleeing until he almost reached the North Korean side.

This spring's annual US-South Korea drills have incorporated fighter jets and nuclear-capable stealth bombers. The allies insist they are routine exercises. Pyongyang calls them rehearsals for a northward invasion and says it needs nuclear weapons to defend itself.